White noise vs Black body radiation

Hi all:
I am confused about one question. When we detect signal from human brain using coil in Nuclear Magnetic Resonance, two source contribute noise to our signal. One is coil itself. The other is human brain. Why can we think the noise from human brain is white noise? Can we compare the noise generated by human brain with that emitted from black body radiation? I know for a resistor, we consider it as approximation black body for current. I hope to know what cause white noise of human brain? thanks for your help.